The trigonometric functions describes relations among angles in a triangle. See the following image:

We see the triangle consisting of three vertices *A*, *B* and *C*. We commonly refer to such triangle as *triangle ABC* or

*△ABC*. Each pair of vertices forms a

*side*in the triangle. We have three sides in the triangle:

*AB*,

*BC*and

*AC*. As you can see the side

*AB*has an alternative name “

*c*”. This name is derived from the only vertex that is not part of the side. So the side

*BC*has an alternative name “

*a*” since the vertex

*A*is not part of the side

*BC*. We use small letters for such alternative names.

Each triangle has three inner angles. By convention we use Greek letters *α* (alpha), *β* (beta) and *γ* (gamma) to mark these angles. The angle *α* is next to the vertex *A* etc. See the following picture.

We can use trigonometric functions in any triangle but often we use trigonometric metric functions in right-angeled triangles. A right-angeled triangle is a triangle in which one angle is a right angle. Right angle means 90-degree angle. See the example:

In the right-angeled triangle each side has it’s own name. The longest one is called *hypotenuse*. The hypotenuse is always on the opposite side than the right angle. The other twe sides are called *legs*. The two legs always forms the right angle and are always shorter than the hypotenuse.